1. Field of the Invention
The present invention relates to a variable speed gearing in a rotary electric tool such as a motor-driven screw-driver or drill.
2. Discussion of Background Information and Prior Art
In conventional motor-driven screw-drivers and drills, the number of revolutions of a terminal output shaft is generally varied high and low by changing the combination in the number of teeth of spur gears. An example is as disclosed in Japanese Patent Publication No. 15764/1984. In such conventional device, however, the change of speed is performed manually using a speed changing handle, so there is inconvenience, which is inconvenient because the operation for change-over must be done during rotation.
More particularly, when the load exerted on the tool in use becomes large and the rotation stops, the speed is changed to low by operating the speed changing handle to increase the tightening torque. The degree of rotation of the tool is checked visually or by the hands which hold the tool, and the speed is changed manually according to the thus-confirmed condition.